Dear Tom,
Could you explain what an “upper-level low” is?  You often mention it when you talk about the weather.

Sam S. Prospect Heights

Dear Sam,
An upper-level low–more precisely, a closed upper low–is a pool of relatively cold air in the atmosphere, generally above 15 thousand feet, about which the wind circulation forms a closed, nearly circular, loop. When viewed from above, wind circulation (in the Northern Hemisphere) is counterclockwise.
By relatively cold it is meant that temperatures in the cold pool are lower than air temperatures at the same height in the surrounding atmosphere. The wind pattern is huge–generally 800-1,500 miles in diameter, sometimes (though rarely) twice that. Upper lows are important because ground-level weather beneath them is invariably cloudy, windy and cold for the season, and usually also wet (rainy or showery in summer, snowy in winter).